1n4d
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The Ligand-Free Structure of E coli BtuF, the Periplasmic Binding Protein for Vitamin B12
OverviewOverview
BtuF is the periplasmic binding protein (PBP) for the vitamin B12, transporter BtuCD, a member of the ATP-binding cassette (ABC) transporter, superfamily of transmembrane pumps. We have determined crystal structures, of Escherichia coli BtuF in the apo state at 3.0 A resolution and with, vitamin B12 bound at 2.0 A resolution. The structure of BtuF is similar to, that of the FhuD and TroA PBPs and is composed of two alpha/beta domains, linked by a rigid alpha-helix. B12 is bound in the "base-on" or vitamin, conformation in a wide acidic cleft located between these domains. The, C-terminal domain shares structural homology to a B12-binding domain found, in a variety of enzymes. The same surface of this domain interacts with, opposite surfaces of B12 when comparing ligand-bound structures of BtuF, and the homologous enzymes, a change that is probably caused by the, obstruction of the face that typically interacts with this domain by the, base-on conformation of vitamin B12 bound to BtuF. There is no apparent, pseudo-symmetry in the surface properties of the BtuF domains flanking its, B12 binding site even though the presumed transport site in the previously, reported crystal structure of BtuCD is located in an intersubunit, interface with 2-fold symmetry. Unwinding of an alpha-helix in the, C-terminal domain of BtuF appears to be part of conformational change, involving a general increase in the mobility of this domain in the apo, structure compared with the B12-bound structure. As this helix is located, on the surface likely to interact with BtuC, unwinding of the helix upon, binding to BtuC could play a role in triggering release of B12 into the, transport cavity. Furthermore, the high mobility of this domain in free, BtuF could provide an entropic driving force for the subsequent release of, BtuF required to complete the transport cycle.
About this StructureAbout this Structure
1N4D is a Single protein structure of sequence from Escherichia coli. Full crystallographic information is available from OCA.
ReferenceReference
Crystal structures of the BtuF periplasmic-binding protein for vitamin B12 suggest a functionally important reduction in protein mobility upon ligand binding., Karpowich NK, Huang HH, Smith PC, Hunt JF, J Biol Chem. 2003 Mar 7;278(10):8429-34. Epub 2002 Dec 4. PMID:12468528
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